This application relates to a control for a heating, ventilation and air conditioning (HVAC) system wherein the control takes in demands from several zones, and determines which of several capacity stages are appropriate given the existing demands. In particular, the present inventive control better meets the demand in that it considers the existence of demand over time in determining whether to change the stage.
HVAC controls are becoming more sophisticated. A basic HVAC control is operable to take in a requested temperature, and compare a requested temperature, or set point, to an actual temperature. The difference is known as the demand. A control then controls the heating or cooling equipment to meet that demand.
More sophisticated systems have several zones, each of which may have an individual demand. As an example, several rooms within a building may each have a set point control that allows a user to select a desired temperature for that particular zone. Each zone may have a sensor to sense the actual temperature. The difference between the desired temperature, or set point, and the actual temperature is known as the zone demand. The demands from the several zones are sent to a control, and the control evaluates how to meet those several demands.
One other feature of modern HVAC systems is that the heating or cooling equipment has several available capacity stages. To ensure the most efficient operation, the control would tend to operate the heating or cooling equipment in the lowest stage that can adequately meet the demand. As the demand increases, then the stage would also increase. Examples of stages might be a furnace provided with several optional heating elements, or an air conditioning system that can be operated in several different capacity modes.
In the prior art, a control for determining the recommended stage looks at a variable known as the system demand. In this prior art control, the system demand is taken from the various zone demands across the system, and calculated utilizing a particular formula. The system demand is considered by a stage control algorithm that selects a desired stage based upon the system demand. The stage control algorithm takes the system demand and multiplies it by some multiplier. The output of this multiplication is a requested stage number. Thus, as the system demand increases, the indicated stage would also increase.
This known control is well suited for low load operations, and when operating at low stages. However, the known control has the potential problem of allowing “droop.” Droop occurs when the system is unable to fully meet the demand, but is within a degree or two of the set points. Particularly when operating at a high stage with the known control, a few degrees difference may never be sufficient to move into the next higher stage operation. Thus, the system can operate for long periods of time without ever fully meeting the demand. It would be desirable to provide a stage control algorithm that is better suited for actually meeting the demand.
One simple HVAC system does include a control for determining a recommended stage that looks not only to a current demand, but also to the existence of a demand over time, or the integral of the demand. This control, however, has not been associated with a multiple zone HVAC system. Instead, the known control only provided a control in a single zone system.